1. Field of the Invention
The present invention relates to a material for resin teeth usable as foreteeth and molars, which is much more improved in terms of mechanical properties and serves to prevent a color change and coloration of the so-called hard resin teeth heretofore available and to improve the adhesion between the enamel and dentinal segments thereof, and a method for making such material for the resin teeth.
2. Prior Art
Both enamel and dentinal segments of conventional resin teeth are composed primarily of polymethylmethacrylate. The components of the resin teeth which are composed primarily of polymethylmethacrylate before they are formed into products are broken down into powdery and liquid components.
The constituents of the powdery component are granular polymethyl methacrylate, polymerization initiator, colorant and so on, while those of the liquid component are methyl methacrylate, polymerizable compound containing at least two ethylenically unsaturated double bonds, and the like. Two such components are mixed together, thereby swelling the granular polymethyl methacrylate with the methyl methacrylate into pasty state, and the thus formed paste is filled under pressure in a counter-die of a denture mold for thermal polymerization. The thus obtained resin teeth are formed of a material stable with respect to some heat or an organic solvent, since it turns to a heat-curing resin crosslinked with the polymerizable compound containing at least two ethylenically unsaturated double bonds. However, a certain limitation is placed on the mechanical properties of such resin teeth, since they are primarily composed of polymethyl methacrylate. The so-called hard resin teeth currently used in incorporated technically with about 20 to 30% of an inorganic filler with the application of an ultra-fine granule type of composite resin material, thereby improving the mechanical properties of their enamel segments. In most cases, the matrix resins used are a polyfunctional methacrylate with no urethane bond but with a phenyl group and containing at least two ethylenically unsaturated double bonds such as, for example, bisphenol A diglycidyl methacrylate and a polyfunctional methacrylate with both an urethane bond and a phenyl group and containing at least two ethylenically unsaturated double bonds such as, for instance, dimethacryloxyethyl-1,2-dimethylphenyl dicarbamate.
In conventional methods for making resin teeth and the so-called hard resin teeth, incomplete polymerization is followed by complete polymerization to prevent air bubbles from occurring in the interiors of the enamel and dentinal segments due to the heat of polymerization generated at the time of polymerization, thereby ensuring sufficient adhesion of the enamel segments to the dentinal segments. More specifically, the lowest temperature and shortest time required for initial pre-polymerization of the methylmethacrylate or polymerizable compound containing at least two ethylenically unsaturated double bonds are first selected. Then, one of the enamel and dentinal segments is subjected to initial pre-polymerization, viz., incomplete polymerization at a temperature of 60.degree. to 70.degree. C. for 15 to 30 minutes. Subsequently, the one of the enamel or dentinal segments is laminated on the other enamel or dentinal segment, followed by incomplete polymerization again at a temperature of 60.degree. to 70.degree. C. for 15 to 30 minutes to bond both segments together. Finally, complete polymerization is carried out at 90.degree. to 100.degree. C. for 15 to 60 minutes.
The materials of resin teeth and so-called hard resin teeth so far used and the methods for making them involve the following problems.
(1) Essentially, the conventional resin teeth composed primarily of polymethyl methacrylate have a surface Knoop hardness of 15 to 20, this is much lower than 300 and 65 for the enamel and dentin of natural teeth, 463 for porcelain teeth, 90 for amalgam, 70 for silicate cement and 35-50 for composite resin. Thus, not only the enamel but also the dentinal segments may wear away, suffer damage and deform due to attrition and strong chewing during mastication or repeated contact with a toothbrush.
(2) The conventional resin teeth composed mainly of polymethylmethacrylate are so stable at a temperature lower than about 80.degree. to 100.degree. C. that they are very unlikely to melt, since they are crosslinked with the polymerizable compound having at least two ethylenically unsaturated double bonds. However, it is likely that a considerably elevated temperature exceeding 160.degree.-200.degree. C., that is the melting point of polymethyl methacrylate, may be instantaneously reached at the time when milling-in is carried out using a carborundum point or a stamp bar with a dental engine for the purpose of occulusal equilibration. Molten resin teeth deposit on such polishing members, there is an extreme reduction in the efficiency of polishing.
(3) In the process of making dentures, a wax model for dentures is invested with gypsum in a dental flask and is then cast to bond resin teeth to a plate resin by thermal polymerization. In this case, since the resin teeth have a coefficient of thermal expansion of as high as 80 to 100 mm/mm/.degree.C., they are brought in firm contact with the gypsum which shows little or no thermal expansion at the temperature at which the plate resin is polymerized. Thus, the rough surface of gypsum is impressed as such onto the resin teeth which, as a result, lose glaze.
(4) Some of the conventional so-called hard resin teeth are formed of a composite material in which a resin matrix composed mainly of a polyfunctional methacrylate with both an urethane bond and a phenyl group and containing at least two ethylenically unsaturated double bonds such as, for instance, dimethacryloxyethyl-1,2-dimethylphenyl dicarbamate, is combined with an inorganic filler in ultra-finely divided form. Such a composite material suffers color change and coloration in the oral cavity within a short period of time due to natural or synthetic food colors, and is often clinically detected as deposits. Thus, clinical results with such teeth are found unfavorable.
(5) Referring to the so-called hard resin teeth prepared with a composite material in which a resin matrix composed mainly of a polyfunctional methacrylate with no urethane bond but with a phenyl group and containing at least two ethylenically unsaturated double bonds such as, for instance, bisphenol A digylcidyl methacrylate or an unsaturated polyester, is combined with an ultra-finely divided inorganic filler, they are generally very short of transparency which is an important factor for artificial teeth. Hence, the teeth assume clouding upon wetted with saliva or water owing to their refractive indices, since their enamel segments are of no depth, and are so clinically and aesthetically not desirable. Inferior transparency also makes it impossible to produce colorful tones of variety.
(6) The enamel segments of the conventional so-called hard resin teeth are formed of a composite material in which a resin matrix composed mainly of a polyfunctional methacrylate with both an urethane bond and a phenyl group or with no urethane bond but with a phenyl group and containing at least two ethylenically unsaturated double bonds is combined with an ultra-finely divided inorganic filler, and so show very inferior adhesion of the polymethyl methacrylate onto the dentinal segments. For that reason, both the segments are bonded together with no incorporation of an inorganic filler into the dentinal segments, i.e., at the cost of the mechanical properties of the dentinal segments. Therefore, if the enamel segments are polished off to the dentinal segments when milling-in or occlusal equilibration is clinically carried out, then any improvement in physical properties such as wear resistance is not attained, since their unhardened portions are exposed to view.
(7) In conventional methods for making the conventional resin teeth and so-called hard resin teeth, incomplete polymerization is followed by complete polymerization to prevent any air bubbles from occurring in the interiors of the enamel and dentinal segments due to the heat of polymerization generated at the time of polymerization, thereby allowing both segments to be sufficiently bonded together. The making time required is a total of 2 hours, say, 30 minutes for the incomplete polymerization of one of the enamel and dentinal segments+30 minutes for the incomplete polymerization of the other+60 minutes for the complete polymerization of both segments.